Russia isn't slow because it's methodical, it's just throwing all of the bodies it can buy at the problem, and the small advancements they get is what they buy with tens of thousands casualties per month. This is doing significant macroeconomic damage to Russia.

Years to capture Donbass is an absolutely pathetic rate of advance for an army that was supposed to be second in the world, fighting a war on its border.

Ukraine is large and conquering all of it would be a massive challenge for any army, but relative to resources Russia has available to it, its performance is awful.